Divalent metal stannate products

ABSTRACT

A divalent metal stannate product having a loss on ignition of 2-17% by weight, e.g. a product intermediate ZnSn(OH) 6  and ZnSnO 3  has advantageous flame-retardant properties, for use e.g. in polymer formulations.

Divalent metal stannate products are being increasingly used asflame-retardant additives in polymer formulations. They generally existin two forms of formula MSn(OH)₆, and MSnO₃, the latter being readilyformed by heating the former to drive off water, and also in a thirdform of formula M₂SnO₄, where M represents the divalent metal. Thus forexample zinc hydroxy stannate (ZnSn(OH)₆) and zinc stannate (ZnSnO₃)have been marketed since 1986 under the trademarks Flamtard H andFlamtard S. Flamtard S is made by heating Flamtard H at 400° C. for asufficient time to drive off the combined water. Flamtard S commands apremium price—but it has superior thermal stability and is recommendedfor high temperature formulations.

This invention results from the discovery that an intermediate producthas properties which are in some respects better than either Flamtard Hor Flamtard S. This discovery was surprising. It was expected that anintermediate product would have intermediate properties.

The invention provides in one aspect a divalent metal stannate productwhich has a loss on ignition of 2-17% by weight, said product inpackaged form for use as a flame-retardant additive. Preferred divalentmetals, on account of the economic importance of their stannates are Zn,Mg, Ca, Ba and Bi; most preferred are Bi and particularly Zn.

In compounds where the atomic ratio of the divalent metal to tin is 1, adivalent metal hydroxy stannate has the formula MSn(OH)₆. On heating at400° C., this product is converted to the corresponding divalent metalstannate MSnO₃ and gives off 3 moles of water per mole of metalstannate. The term “metal stannate product” is herein used to cover themetal stannate and partially hydrated compounds not including the fullyhydrated divalent metal hydroxy stannate.

The loss on ignition of a compound is determined by first drying thecompound at 110° C. to constant weight, then heating the compound at1000° C. to constant weight, and noting the percentage differencebetween the two. The loss on ignition of divalent metal hydroxystannates depends on the atomic weight of the divalent metal. Forexample, the loss is theoretically 18.9% for zinc hydroxy stannate and12.6% for bismuth hydroxy stannate. The loss on ignition of a divalentmetal stannate is in principle zero but may in practice be up to 1.5%.

The divalent metal stannate products of the present invention arecharacterised by having a loss on ignition of at least 2% preferably atleast 4%; but less than the corresponding divalent metal hydroxystannate. For zinc stannate products the loss on ignition is preferably2-17% e.g. 4-15%. For bismuth stannate products the loss on ignition ispreferably 2-11% e.g. 4-9%. Preferably the divalent metal stannateproduct has a formula MSnO₃.xH₂O where x is 0.4-2.6 e.g. 0.8-2.2.

These divalent metal stannate products may be made by heating acorresponding divalent metal hydroxy stannate under conditions to driveoff a desired proportion of the combined water present. Suitable heatingtemperatures are 200-350° C. It may be convenient to use the same kindof rotary kiln as is currently used for converting Flamtard H toFlamtard S, e.g. for 2-40 minutes. Of course, that process produces atransient partially dehydrated zinc stannate product, but that transientproduct is in commercial production never recovered and packaged for useas a flame-retardant additive.

Or the metal hydroxy stannate may be heated in a spin flash dryer,preferably at a temperature from 130° C. to 400° C. Or the metal hydroxystannate may be heated in a static oven e.g. for 30 minutes to 5 hours.Different heating regimes give rise to slightly different products whichmay be optimum for different purposes. In general, the heatedintermediate product of this invention is white (like Flamtard H) ratherthan pale yellow (like Flamtard S or a mixture of H and S).

Alternatively the divalent metal stannate products of this invention maybe made simply by mixing together the corresponding divalent metalhydroxy stannate and divalent metal stannate in suitable proportion. Forexample, 90−10 wt % of Flamtard S may be mixed with 10-90 wt % ofFlamtard H. This simple mixture has properties which, while generallynot as good as those of the heated products, are neverthelesssurprisingly superior to both the Flamtard H and the Flamtard S whenused separately.

In another aspect the invention provides a polymer formulationcontaining an effective concentration of a flame-retardant additivewhich is a divalent metal stannate product as defined, either alone orin admixture with a divalent metal borate. For example, such a mixturemay consist of 90% to 10% by weight of the divalent metal stannateproduct and correspondingly 10% to 90% by weight of the divalent metalborate. Zinc hydroxy stannate and zinc stannate may be added to thepolymer formulation either separately or together as a pre-mixture.

Polymer formulations may be those in which zinc (or other metal) hydroxystannate and zinc (or other metal) stannate are currently used asflame-retardant additives. These include polyurethanes both solid andfoam formulations, polyethylene particularly linear low densityproducts, polypropylene, natural and synthetic rubber latex, polyamide,polystyrene, epoxies, neoprene, phenolics, EPDM/EVA blends, andparticularly polyvinylchloride formulations. When used in concentrationsof 1-30% more usually 5-20%, by weight on the weight of the polymer,these additives provide useful flame-retardant properties.

The following examples relate to intermediate zinc stannate products andblends which are shown to be superior to Flamtard H (commercial zinchydroxy stannate) and Flamtard S (commercial zinc stannate) in certainrespects:

Better critical oxygen index values (for both normal COI and elevatedCOI).

Electrical properties (improved volume resistivity).

Improved general fire performance (cone calorimeter data).

Reduction in smoke (NBS smoke chamber and cone calorimeter data).

It may be inferred that corresponding improvements would be shown byother divalent metal stannate products such as bismuth stannateproducts.

EXAMPLE 1

Commercial zinc hydroxy stannate was heated at 300° C. in a laboratoryoven for 1.5 hours or 3 hours, to give zinc stannate products accordingto the invention having a loss on ignition between 4.05% and 16.63%. Theproducts were used as flame-retardant additives in the following polymerformulation (in which figures are in parts by weight per 100 parts byweight of polymer or phr):

PVC DS7060 100 PhospHate plasticiser (Santicizer 148) 30 Brominatedaromatic ester (DP45) 20 Alumina trihydrate (SF4E) 30 Stabiliser(Irgastab 17M) 7 Stearic acid (Processing aid) 0.5 Irgawax 371(Processing aid) 0.5 Flame retardant (various) 10

The formulations were compounded in a two roll mill at 140° C. andmoulded at 150° C. Plaques of 2 mm and 3 mm were moulded for 1 minuteunder no pressure then 3 minutes under 3000 MPa.

Mechanical properties were tested on a Zwick machine according toBS6469. COI was tested for each sample according to BS2782. Irradiancefor cone calorimetry test was 40 kW/m² for all samples except Flamtard Swhich was tested at 70 kW/m². The results are set out in Table 1.

TABLE 1 Flamtard Flamtard Commercial Commercial FORMULATION HEATED 1.5 hHEATED 3 h Flamtard S Flamtard H IGNITABILITY Time to Ignition (s) 72 5822 56.5 HEAT RELEASE Peak HRR (kW/m²) 134.7 126.6 147 128.5 Time to PeakHRR (s) 200 205 130 131 Total HR (MJ/m²) 49.8 47.9 50.5 45.43 SMOKE PeakRSR (1/s) 8.31 8.99 15.82 13.21 Total Smoke Release 2,017 2,051.23,120.6 3,242 Average SEA (m²/kg) 566.1 549.9 871.6 910.2 GAS PRODUCTIONPeak CO (kg/m²s) × 10³ 1.35 1.33 1.62 1.58 Peak CO₂ (kg/m²s) × 10³ 7.886.95 7.15 7.9 FPI (s m²/kW) 0.53 0.46 0.15 0.44 MECHANICALS Elongation @Brk (%) 228.8 238.4 163.84 152.3 Tear Strength (N/mm) 13 12.5 11.69 12.0COI (% O₂) 40.1 40.3 40 39

EXAMPLE 2

Various flame-retardant additives were mixed with already compoundedEPDM/EVA blends. The formulations were mixed on a two roll mill at 70°C. Then the resulting compounds were compression moulded at 170° C. for25 minutes under pressure into different thickness plaques. Theflame-retardant additives were:

A and B Flamtard H (commercial Zinc hydroxy stannate) added at 10 phrand 15 phr.

C A zinc stannate product according to the invention made by heatingFlamtard H at 300° C. for 3 hours as described in Example 1, and used at10 phr.

D A blend of 6 phr of the said zinc stannate product with 4 phr of zincborate.

The following tests were performed: COI and elevated temperature COI;and NBS smoke chamber. The results are set out in Table 2.

TABLE 2 FORMULATION A 10 phr B 15 phr C D COI (% O₂) 55.7 57.7 60.3 60.3Elevated 290 300 300 320 temperature COI (° C.) Specific Optical 130 137127 126 Density

EXAMPLE 3

Commercial zinc hydroxy stannate was heated at 250° C. for 2 hours, thencooled in a dessicator and introduced into a PVC formulation as follows.Commercial zinc stannate Flamtard S was used in a control formulation.The formulation was (in phr):

PVC (DS7060) 100 Ca/Zn stabiliser 5 Kaolin 5 CaCO₃ 40 Cereclor S52(chlorinated aromatic) 13 Dioctyl phthalate 40 Flame retardant 5

Mixtures were compounded in a two roll mill at 140° C. then compressionmoulded at 150° C. for 1 minute and then 3 minutes under pressure.Samples were tested in COI and volume resistivity (Digital SuperMehohmmeter DSM-525A) and expressed in (M ohm.km). Cone calorimetryanalysis was performed with an irradiance of 50 kW/m² with edge frameand grid. The results are set out in Table 3

TABLE 3 Flamtard Heated 250° C., FORMULATION Commercial Flamtard S 2hours Time to ignition (s) 23 25.5 Peak of heat released (kWm⁻²) 157.5136.5 Tota heat released (MJm⁻²) 91.05 86.34 Specific extinction area(m²kg⁻¹) 753 682 COI (% O₂) 29.9 31.1 Voiume resistivity, K value 430.7954 (Mohm.km)

EXAMPLE 4

The following Table 4 gives surface area and particle size data forvarious materials. Flamtard H and Flamtard S are the commercialproducts. 250, 285 and 310 are products formed by heating Flamtard H at250° C., 285° C. and 310° C. respectively.

TABLE 4 Surface Area Median Particle m²/g Size μm Flamtard H 14.01.5-3.0 Flamtard S — 1.5-3.0 250 17-35 1.5-3.0 285 31-73 1.5-3.0 31046-65 1.5-3.0

EXAMPLE 5

The following formulation was made up (amounts in g):

PVC (DS7060) 195 Dioctyl phthalate 81 ESO 10 Stabiliser (Irgastab EZ712) 10 Huntite 72.4 Aluminium trihydroxide (SF4E) 75.2 Clay (Whitex)74.6 Flame reatardant (varies) 28.6

As flame retardants were used: Flamtard H; Flamtard S; and a product(HS-250 (2)) made by heating Flamtard H in a laboratory oven for 2 hoursat 250° C.

Plaques for testing were made as described in Example 1. Irradiance forcone calorimetry testing was at 40 kW/m². Results are set out in Table5.

TABLE 5 Flame Retardant H S HS-250(2) Time to Ignition (secs) 84 74.585.5 Peak rate of heat release (kW/m²) 87 122 78 Average specificextinction area (m²/kg) 229 297 157 Total smoke released 800 1089 635Fire Performance index (m²s/kW) 0.96 0.62 1.10 Smoke Parameter (MW/kg)19.9 36.0 12.2

All these properties of HS-250(2) were distinctly better than of eitherH or S.

EXAMPLE 6

The following formulation was made up (parts are per hundred of PVC):

PVC 100 Ca/Zn stabiliser 5 Kaolin 5 CaCO₃ 40 Cereclor S52 13 DIDP 40Flame retardant 5

The following flame retardants were used:

Flamtard H (zinc hydroxystannate)

Flamtard S (zinc stannate)

T-28FD—Flamtard H heated at 280° C. in a spin flash dryer.

T-28-4RC—Flamtard H heated at 280° C. for 4 minutes on a rotarycalciner.

T-28-25RC—Flamtard H heated at 280° C for 25 minutes on a rotarycalciner.

Critical Oxygen Index and electrical resistivity (k values) propertiesafter 2 days are reported in Table 6.

TABLE 6 Property COI (% O₂) κ Value (Mohms.km) Flamtard H 29.8 243Flamtard S 29.1 162 T-28FD 29.3 290 T-28-4RC 29.9 551 T-28-25RC 29.3 405

The electrical properties of H and S are distinctly inferior to theothers.

EXAMPLE 7

The following formulation was made up (parts per hundred of PVC):

PVC (DS 7060) 100 DOP 30 Cereclor S52 20 Calcium carbonate 60 CD33 (EZ712) 5 Flame retardant 5 (when used)

The flame retardants used were: Flamtard H; Flamtard S; and a 50:50mixture of the two (H/S (5:50)).

Procedure:

Five samples were compounded following the formulation shown above usinga Two-Roll-Mill at 140° C. 2 and 3 mm plaques were compression mouldedat 150° C. for 1 minute close and then 3 minutes under 20 tonnespressure. Plaques were rested for 16 hours before testing.

10 small tensile dumbbells and 10 tear test pieces were cut from the 2mm thickness plaque to test their mechanical properties. Tensile testwas carried out under 250 mm/min speed and 5N pre-load. The speed fortear resistance test was 400 mm/min with a 5N pre-load.

Cone calorimeter testing was performed using a 40 kW/m² heat flux.Results are set out in Table 7.

TABLE 7 Flame Retardant None H/S (50:50) H S Time to Ignition (s) 4551.5 48.5 51.5 Peak Heat Release (kW/m²) 169.3 145.1 144.5 154.2 Time toPeak HRR (s) 152.5 180 170 167.5 Peak rate of smoke release (1/s) 12.579.32 9.35 10.43 Total smoke release 2646 2281 2378 2337 Fire performanceindex (sm²/kW) 0.266 0.355 0.336 0.334 Tensile Strength at Yield (MPa)15.8 16.9 16.2 15.2 Elongation at Yieid (%) 166.4 173.5 172.9 168.8

The performance of H/S (50:50) was somewhat better than of H or S.

EXAMPLE 8

The PVC formulation and processing were as in Example 7. The flameretardants used were: Flamtard H; Flamtard S; and a product HS-250(1)made by heating Flamtard H for 1 hour at 250° C.

Critical oxygen index (COI) was tested following BS 2782 in 3 mmthickness and 10 mm wide strips.

Specific optical density was determined using NBS smoke box with sampledimensions of 75×75×3 mm.

Results are set out in Table 8.

TABLE 8 Flame Retardant None H S HS-250 (1)4 Tensile Strength at Yield(MPa) 15.8 16.2 15.2 16.3 Elongation at Yield (%) 166.4 172.9 168.8190.4 Specific Optical Density (D_(max)) 366 291 311 284 COI (% O₂) 28.831.9 29.4 32.0

The properties of HS-250(1) are better than those of H or S.

EXAMPLE 9

The following formulations were made up (in % by weight):

1 2 Nylon 79 79 Dechlorate plus 25 15 15 Flamtard S 6 HS-250(2) 6

HS-250(2) was made by heating Flamtard H for two hours at 250° C.

Samples were subjected to a 125 mm vertical burn test with a UL-94-5VAclassification. Both formulations passed the horizontal requirements ofthe test. Formulation I failed, but formulation 2 passed the verticalrequirements.

EXAMPLE 10

Flamtard H was compared to HS-300 (3) (obtained by heating Flamtard Hfor 3 hours at 300° C.) in an EPDM/EVA flooring compound. The exactformulation is not known, but it probably contained a large proportionof aluminium trihydroxide. Results are set out in Table 9.

TABLE 9 10 phr Flamtard H 10 phr HS-300 (3) COI (% O₂) 55.7 60.3Specific optical density 130 127

The COI figure for HS-300(3) is considerably better than H.

What is claimed is:
 1. A divalent metal stannate product which has a loss on ignition of 2-17% by weight, but less than the loss on ignition of the corresponding divalent metal hydroxy-stannate, said product in packaged form for use as a flame-retardant additive.
 2. A divalent metal stannate product as claimed in claim 1, having the formula MSnO₃.xH₂O where M is a divalent metal and x is 0.4-2.6.
 3. A divalent metal stannate product as claimed in claim 1, wherein the divalent metal is Zn.
 4. A divalent metal stannate product as claimed in claim 1, which is a mixture of 90−10 wt % of zinc hydroxy stannate with 10-90 wt % of zinc stannate.
 5. A polymer formulation containing an effective concentration of a flame-retardant additive which is the divalent metal stannate product according to claim
 1. 6. A polymer formulation containing an effective concentration of a flame-retardant additive which consists of 90% to 10% by weight of the divalent metal stannate product according to claim 1 and correspondingly 10% to 90% by weight of a divalent metal borate.
 7. A polymer formulation as claimed in claim 5, wherein the flame-retardant additive is present in a concentration of 1-30% by weight.
 8. A method of making the divalent metal stannate product of claim 1, which method comprises heating a corresponding divalent metal hydroxy stannate at a temperature of 200-350° C.
 9. A method as claimed in claim 8, wherein zinc hydroxy stannate is heated for 30 minutes to 5 hours.
 10. A polymer formulation containing an effective concentration of a flame retardant additive which consists of 90−10 wt % of zinc hydroxystannate with 10-90 wt % of zinc stannate.
 11. A polymer formulation as claimed in claim 6, wherein the flame-retardant additive is present in a concentration of 1-30% by weight.
 12. A method of making the divalent metal stannate product of claim 1, which method comprises mixing together a corresponding divalent metal hydroxy stannate and divalent metal stannate in suitable proportions. 